In recent years we have begun using large in line exhaust fans for large or multiple residential bathrooms. Often times one fan can effectively work for 2 bathrooms.

We do not install 3-way switches for the fans because sometimes the fans are so quiet that you cannot tell if it is on or off. Instead we will install 2 single pole switches , 1 per bathroom and wire them in parallel so that if either switch is on the fan will operate. I know this is common practice with many other contractors but I'm wondering if it is illegal? Could this be a violation of 310.4? I have never been flagged for this configuration.

Possible safety hazard.You turn off the switch in the bathroom and work on the fan. Someone turns on a switch in another bathroom and you have a problem.Not sure of the code violation but, seems like a hazard.Alan--

By 'where is the exception that allows this?' are you asking for an exception to 310.4 Conductors in Parallel?

Although with both switches on there are _two_ paths that current could follow, and thus this is a parallel circuit, I do not believe that these qualify as conductors in parallel for the purposes of 310.4. The last sentence of 310.4 reads '...shall be permitted to be connected in parallel (electrically joined at both ends to form a single conductor).' In this case, the conductors to and from the various switches are _not_ joined to form single conductors, but are _separate_ circuit pathways.

IMHO the electrical difference between the multiple switches that we are discussing and a ring circuit is the 'loop area'. Loop area means inductance in the circuit where substantial energy can be stored in a magnetic field. This means increased circuit inductance and production of external EMFs. Switch loops do not add 'loop area', because current flow in one conductor is balanced by current flow in a matching adjacent conductor.

In the UK, 'ring circuits' are specifically permitted to be protected at a higher current level than 'spur' circuits of the same gage. The NEC has no provision for this, so this 'benefit' of ring circuits would not be found in an NEC compliant installation.

In the US, a 'ring circuit' would need to comply with 300.3(B); in a ring circuit you _explicitly_ don't have all conductors of the same circuit in the same raceway.

I believe that a ring circuit, using non-metallic wiring methods, and protected as is normal for the given conductor size, would meet the requirements of the NEC. I also think that it would be a bad idea from the point of view of emf issues, and would offer little benefit because circuit ampacity could not be increased.

Thinking about it some more, I belive that a multiple switch installation could easily run afoul of 300.3(B). If you ran a feeding circuit to a couple of switches, and then separate switched feeds from the multiple switches to a single outlet, then not only would you have parallel electrical paths, but you would also have conductors of the same circuit in different raceways, as current could go through one switch leg but a different neutral conductor. IMHO this would again be allowed using non-metallic wiring, but a bad idea because you are introducing large loop area in the current flow pathway.

On the other hand, if you ran all of the switches as switch loops, or ran one switched feed and the other switches on a loop of that first switch, then I wouldn't see a problem. 300.3(B) does not explicitly permit switch loops, but switch loops are clearly allowed. Running these switches as switch loops does not introduce significant loop area.

Actually you could run two #12s from a 20 amp CB to a single receptacle.

The parallel conductor rule is for the protection of the paralleled conductors. It allows the OC device to be much higher than the required level for any single conductor. If one follows the rules for parallel conductors, the current will divide equally to all conductors, and the OC device will provide the required protection for the paralleled conductor assembly. If the rules are not followed, then current flow will not be equal in each paralleled conductor, and one may carry much more current than the others, and be overheated, damaging the insulation by heating it beyond its temperature rating.

However, if the OC device is low enough to be sufficient to protect any single conductor, then go ahead string your circuits such that there may be several simultaneous paths from source to load (parallel paths). All conductors are protected from overheating, no damage can occur, and the intent of the NEC is maintained. (not withstanding the appliance or motor disconnect rules).

There is a difference between paralleled conductors and switching arrangements in parallel.

Ring circuit arrangements are a different story. There is a NEC requirement to run all conductors of a circuit together to eliminate inductive loops. That is why we only find ring circuits on overhead spans of utility distribution lines.